Anal. Chem. 1991, 63,993-1000
with longer beam lines. Mass spectrometrists may be interested in exploring the technique's potential for high resolution. By pushing the technique with faster rise times of the BVM wave form, higher beam voltages, smaller BVM tubes, and energy selection of the continuous ion beam, vastly higher resolutions could be achieved. The technique shows great promise as a method for high-frequency population modulation and appears to be amenable to Fourier transform mass spectrometric techniques.
ACKNOWLEDGMENT We are also indebted to A. G. Marshall for discussions about this instrument and our neighbors who have lent us equipment, including R. L. McCreery, C. W. Mathews, and T. A. Miller. LITERATURE CITED (1) Coe, J. V.; Saykaiiy. R. J. I n Ion and Cluster Ion Spectroscopy and Stnrcfure; M e r , J. P., Ed.; Elsevier: Amsterdam, 1989; pp 131-154. (2) Coe. J. V.; Owrutsky, J. C.; Keim, E. R.; Agman. N. V.; Hovde, D. C.; Saykally, R. J. J. Chem. Phys. 1989, 90, 3893-3902.
993
(3) Keim, E. R.; Polak, M. L.; Owrutsky, J. C.; Coe, J. V.; Saykally, R. J. J . Chem. Phys. 1990, 93, 3111-3119. (4) Alford, J. M.;Williams, P. E.; Trevor, D.J.; Smalley R. E. Int. J. Mess Spectrom. Ion Roc. 1988. 72, 33-51. (5) Hanson, C. D.; Keriey, E. L.; Russell, D. H. I n Treatise on Analyticel Chemistry; Wlnefordner, J. D.. Ed.; Wlley Interscience: New York, 1989; Vol. 11, Part I, pp 128-133. (6) Cowen, K. A.; Coe. J. V. Rev. Sci. Instrum. WBO, 61. 2601-2604. (7) Cotter, R. J. B i d . Environ. Mass Spectrom. 1989, 18, 513-532. (8) Brunnee, C. Int. J. Mess Spectrom. Ion Roc. 1987, 76, 125-237. (9) Watson, J. T. Introduction to Mess Spectrometry; Raven, New York, 1985. (10) Pinkston, J. D.; Rabb. M.;Watson, J. T.; Allison, J. Rev. Sci. Instrum. 1985, 5 7 , 583. (11) Weber, C. I n Focusing of Cherged Particles; Septier. A., Ed.; Academic Press: New York, 1967; Vol. 1, pp 45-99. (12) Computer program written by Charles Schmuttenmaer. University of California, Berkeley; modified for use on IBM PC clones by J. Coe.
RECE~VED for review November 26,1990. Accepted February 27,1991. We gratefully acknowledge the support of the Department of Chemistry at The Ohio State University as well as the Petroleum Research Fund, administered by the American Chemical Society.
Electrical Double-Layer Model for Sorption of Ions on Octadecylsilyl Bonded Phases Including the Role of Residual Silanol Groups Hanjiu Liu and Frederick F. Cantwell* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Sorption isotherms on a highly end-capped octadecyisiiyi (ODS) silica bonded phase packing have been measured at various ionic strengths, adlusted with NaCi, both for tetra-nbutyiammoniumcation (TBA'), from a solution of its chloride salt, and for p-nitrobenzenesulfonateanion (NBS-), from a solution of its sodium salt, using the column equilibration technique. At constant ionic strengths the NBS- isotherms are strictly Langmuirian, while the TBA' isotherms are Langmulrian only after subtractlng the moles of TBA' that are strongly adsorbed on residual siianoi groups. The completeness of elution of TBA' in the experiment has been verified by neutron activation analysis. Sorption of each of the organic ions, NBS- and TBA', is quantitatively described by the Stern-Guoy-Chapman (SGC) model of the electrical double layer. The surface potential $, is Nernstian for the TBA' system but is markedly sub-Nernstian for the NBS- system. The latter is due to the presence of a small number of anionic siianoiate sites on the ODS packing.
INTRODUCTION The sorption of ions onto chemically bonded reversed-phase stationary phases in liquid chromatography is important both in the direct separation of ions (1)and in the separation of ions by the more popular technique of "ion-pair" chromatography (2). In both techniques the sample ions to be separated are sorbed onto the stationary phase, and in the latter technique this sorption occurs in the presence of "pairing ions", which also are sorbed. The sorption of ions onto nonionic interfaces has been the object of study for many years because of its importance in
surface and electrochemistry and in separation science and technology. On nonpolar sorbents the main chemicul attractive forces between the solute and the sorbent are dispersion forces but, because these are relatively weak, solvent-solute interactions, solvent-solvent interactions, and solvent structure (entropy) effects also account for a significant fraction of the chemical interaction energy ( 3 , 4 ) . For ionic solutes there exists an electrostatic interaction energy in addition to the chemical interaction energy. Nearly all of the well-established physicochemical models for ionic sorption take into account the electrical potential and the electrical double layer that develop at the interface (5-7). This is true for ionic sorption a t air-liquid and liquid-liquid interfaces (8)as well as at solid-liquid interfaces involving both polar and nonpolar solids such as quartz, graphite, and polystyrene (3, 4 , 9). Since the introduction of classical electrical double-layer concepts to explain the sorption of ions on reversed-phase packings used in high-performance liquid chromatography (HPLC) (IO), they have increasingly been invoked to explain ionic sorption on both porous polymers and chemically bonded sorbents (11-27). A form of the Stern-Gouy-Chapman (SGC) theory of the electrical double layer was initially proposed (10). The SGC theory is derived by solving the Poisson-Boltzmann equation for a planar interfacial geometry under semiinfinite conditions (5). In this laboratory, experimental tests of the SGC model have been based on measuring the ionic strength dependence of the distribution coefficient for the ion. Recently, others have proposed modifications to this model, such as solution of the Poisson-Boltzmann equation for different geometries, in an effort to more realistically represent the electrical potential gradient within pores (21,25). The results of these efforts do not give significantly better
0003-2700/91/0363-0993$02.50/00 1991 American Chemical Society
994
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
agreement with the experimental data than is obtained with the planar, semiinfinite model. This is not surprising given the fact that any relatively simple geometry will be a poor approximation of the real surface geometry inside a porous particle. Furthermore, even if an exact description of the geometry were employed, solution of the Poisson-Boltzmann equation would be an Herculean task. Thus, we opt for the most tractable form of the double-layer theory, which, in any event, has already been found to give quite acceptable agreement with experimental results for porous polymer particles (IO,14). Also, others have employed alternative criteria than ionic strength dependence to experimentally test the double-layer model (12,17,21,25,26). While these other criteria may also be valid, we shall continue to use ionic strength dependence because this treatment explicitly includes electrical potential and capacitance, which are the parameters in terms of which SGC theory is traditionally discussed (5-7), and it makes possible a direct test of Nernstian behavior. In the present paper we experimentally investigate the sorption of an organic anion, p-nitrobenzenesulfonate (NBS-), and of an organic cation, tetra-n-butylammonium (TBA+) from aqueous solution onto an ODS bonded phase, Partisil-10 ODs-3,using the column equilibration technique (28, 29). Sorption isotherms are measured a t various ionic strengths, and the data are evaluated in terms of the SGC theory.
EXPERIMENTAL SECTION Chemicals and Solvents. p-Nitrobenzenesulfonic acid was prepared from practical grade material (Eastman Kodak) by recrystallization from benzene/ethyl acetate (1:3 v/v). The crystals thus obtained were vncuum-dried for 24 h (mp 92-93 "C). Tetrabutylammonium chloride (TBA+Cl-)(Eastman Kodak) was used as received. The label-claim was 96%, and the material is very hygroscopic. All other chemicals were reagent grade. Water used in all experiments was demineralized, distilled, and finally distilled over alkaline permanganate. Methanol (Anachemia) and ethanol (BDH Chemicals) were reagent grade and were distilled before use. Sample and Eluent Solutions. For the measurement of NBSisotherms five series of solutions were prepared with water ns solvent. Each series had a different but constant ionic strength and contained a range of NBS- concentrations from 0.050 x lo" to 1.50 X M. The five series were at ionic strengths 0.010, 0.022,0.034,0.050, and 0.100. All of these solutions had their pH adjusted to 5.0 with 1 X lo4 M acetic acid/sodium acetate buffer and had their ionic strengths adjusted by addition of NaCl. The eluent used to elute NBS- from the column after achievement of column equilibration was methanol/water (1:1 v/v). For the measurement of TBA+ isotherms another five series of solutions were prepared with water as solvent. Each series had a constant ionic strength and contained a range of TBA+ conto about 2 X M. The five centrations from about 2 X series were at ionic strengths of 0.050,0.070,0.100,0.300, and 0.500 M. Solution pH was adjusted to 5.0 with 1 X lo-' M acetic acid/sodium acetate buffer and ionic strengths were adjusted with NaC1. The eluent used to elute TBA+ from the column after achievement of equilibration was methanol/water (1:l v/v) containing 0.010 M NaC1. All solutions were filtered through a 0.45-pm pore size Nylon 66 filter (Ranin Instrument Co.) before use. ODS Sorbent. Partisil-10 ODS-3 (Batch No. 100763, Whatman Inc.) is a l0-pm diameter, irregularly shaped, chemically bonded, polymeric octadecylsilyl reversed-phase column packing with 10% carbon loading. It is reported by the manufacturer to be highly "end-capped" with a surface coverage of silanols by bonded groups of at least 95% (30). The specific surface area of this batch of packing was measured, by using the BET method, to be 309 m2/g. Apparatus and Procedure. The column equilibration apparatus has previously been described (28,29). In the present study only the upper half of the apparatus was used, consisting of a pump P1for sample solution (Model 501, Waters Associates),
injection valve V1 (Part No. 7010, Rheodyne Inc.), pump P2 for eluent (Model 590, Waters Associatm), and (pre)columnC1, which contained the ODS packing (see Figure 1, ref 28). Two different designs of the column C1 were used. For the NBS- study the column was a 2.0 cm x 0.40 cm i.d. stainless steel guard column (Part No. 84550, Waters Associates), which was dry-packed with 0.165, g of ODS packing. For the TBA+ study a cartridge column (Part No. 28690, Chrompak, Netherlands) with a 0.20 cm X 0.46 cm i.d. chamber, dry-packed with 1.58 X g of the ODS packing, was used. The column was inserted in place of the injection loop of value V1 and was thermostated at 25.0 f 0.5 "C by immersion in a water bath. In the column equilibration experiment a solution of either NBS- or TBA+ was pumped through the column at a flow rate of 2.0 mL/min until equilibrium was achieved between the ODS packing and the solution (Le. complete breakthrough was achieved). This was the loading step. Valve VI was then switched to allow the eluent, pumped at a flow rate of 1.0 mL/min, to elute the ion of interest from the column into a volumetric flask (to the mark). This was the elution step. The tote1 number of moles nT of the ion of interest eluted from the column, including that in the hold-up volume, was then measured by determining the concentration of the ion in the solution in the volumetric flask by a suitable quantitative technique. For the ion NBS-, the concentration was determined spectrophotometrically at 266 nm on a Cary 118 spectrophotometer (Varian Instruments). For the ion TBA+, the concentration was determined by ion-pair extraction of TBA-picrate (31)using the technique of solvent extraction/flow injection analysis (SE/FIA) (27).The SE/FIA apparatus was very similar in design to that used previously in the determination of the procyclidinium ion (32). The membrane phase separator consisted of two layers of 10-20-pm pore size porous Teflon (Zitex, Part No. E249-122, Chemplast, Wayne, NJ) and the detector was an SP 8200 HPLC detector (Spectra-Physics) set at 365 nm. Important instrument parameters in the SE/FIA determination were 50-pL injection volume, 1.4mL/min total CHC13 flow rate, 1.1 mL/min total aqueous flow rate, 0.5 mL/min CHCI3 flow rate through the membrane phase separator, 1.5-m extraction coil length, and 30 psig nitrogen pumping pressure. The instrument was calibrated by injecting standard solutions containing between 1 x lo4 and 1 x 10-3 M TBA+. The concentrations of the ion NBS- or TBA+ sorbed onto the ODS packing, Cs (mol/kg), was calculated by the equation
in which CM is the molar concentration of the ion in the sample solution pumped by PI, VM is the total hold-up volume (liters) of the column and valve system, measured as described below, and Ws (kilograms) is the weight of ODS packing in the column. The sorption isotherm of the ion is a plot of Cs versus CM and is obtained by repeating the above experiment at several different CM.
It was deomonstrated by varying the sample loading time that column equilibration was achieved for all NBS- solutions in less than 3 min (Le. 6 mL) and for all TBA+ solutions in less than 20 min (i.e. 40 mL). Therefore, to be safe, loading times of 5 and 60 min were always used for NBS- and TBA', respectively. It was also demonstrated, by varying the elution time, that 6 min (Le. 6 mL) was sufficient in the case of NBS- and 4 min (Le. 4 mL) was sufficient in the case of TBA+ for all of the sorbed ion to be eluted. To be safe, 10 and 25 min elution times were always used for NBS- and TBA+, respectively. Measurement of Vw The hold-up volume includes the void volumn of the packed-bed and frits and the volumn of the connecting tubing (28). VM was measured by f i t pumping pure water through the column using sample pump Pl and then switching the valve V1 to elute the water with pure ethanol pumped by P,. The effluent was collected in a 10-mL volumetric flask to which 0.100 mL of methanol was added as an internal standard. The volume of eluted water, VM,was measured by determining the concentration of water in the ethanol by gas chromatography at 135 "C on a 2.9 m X 1.6 mm i.d. column of 50/80 mesh Porapak Q-S (Waters Associates) using a Model 3700 (Varian Associates)
ANALYTICAL CHEMISTRY, VOL. 63,NO. 10, MAY 15, 1991
gas chromatograph with a thermal conductivity detector. The hold-up volumes of the long and small precolumns are 212 f 4 and 65 f 1 rL, respectively, where the uncertainty is the 95% confidence limit. Surface Tension Measurement. Surface tensions were measured by the Wilhelmy plate method (33) for a series of water M TBA+Cl- and solutions ranging from 1.0 X to 2.0 X adjusted to ionic strength 0.50 with NaCl. The platinum plate was 2.593 cm wide and 0.0109 cm thick, and the balance was a Model R-100 electrobalance (Cahn Instruments). The plate was calibrated with pure water prior to use. Neutron Activation Experiment. This experiment was done to demonstrate that TBA+is quantitatively eluted from Partisil-10 ODs-3. The idea was to determine the amount of TBA+ remaining on the column after it had been eluted with 25 mL of the designated eluent 0.010 M NaCl in methanol/water (1:l v/v). Instead of using TBA+, its monobrominated derivative (4bromo-n-buty1)tri-n-butylammonium ion, TBABr+, was used because bromine in this ion could be detected at very low levels on the ODS packing by instrumental neutron activation analysis (INNA) (34). Since TBABr+ is even more strongly sorbed on the ODS packing than TBA+ (27),it is reasonable to assume that if TBABr+ is completely eluted, then TBA+ will also be completely eluted. TBABr+Br-was synthesized by reacting tri-n-butylamine with an excess of l,4-dibromobutane in methanol for 48 h under reflux (35). After workup the product was recrystallized twice from diethyl etherln-hexane and vacuum-dried to yield a white crystalline product with a bromine content of 38.5% (theory: 39.9%)and showing a peak for TBABr+ at m / e 321 by fast atom bombardment mass spectrometry. In the experiment, the small precolumn C1 was first loaded by pumping through it 40 mL of a pH 5.0 solution containing 2.88 X lo5 M TBABr+Br-at ionic strength 0.050 and was then eluted 5 was next with 25 mL of eluent. The ODS packing ( ~ 1 mg) transferred into a polyethylene vial, dried at room temperature, and subjected to neutron irradiation for 5.00 min at a neutron ’ ~ in the SLOWPOKE I1 reactor at the flux of ~ 1 0 n/(cm%) University of Alberta. The sample was allowed to ‘cool” for 2.00 min before counting the 619 keV y-ray peak for 80Br( t l l 2 = 17.7 min) for 5.00 min. Fresh ODs packing was used for each replicate experiment. A blank for the procedure waa prepared by equilibrating an ODS column with a pH 5.0, ionic strength 0.050 solution containing 2.88 X M TBA+Br-(containsno covalently bound Br), eluting and treating as described above. The purpose of the blank was to demonstrate that the inorganic Br- anion is eluted from the column so that any Br detected by INNA is due to TBABr+. Bromine-containing standards for the analysis were prepared by pipetting aliquots of a NHIBr solution onto =15-mg quantities of ODS packing in polyethylene vials, along with two or three drops of methanol to “wet” the packing. After air-drying these standards were subjected to INNA as above. R E S U L T S AND DISCUSSION SGC Theory. The Stern-Gouy-Chapman (SGC) theory, as it applies to the sorption of ions a t the interface between a nonionic sorbent and the solution phase, has previously been described (IO). The sorbed organic ion, TBA+ or NBS-, is a potential-determining ion because it is responsible for the electrical potential difference, $o, between the interface and the bulk solution. Electroneutrality in the vicinity of the interface is maintained by a net surface excess of oppositely charged ions in the solution adjacent to the interface. The following expression applies for symmetrical 1:l electrolytes such as TBA+Cl-, Na+NBS-, and Na+Cl-, a t 25 “C:
2.28 X 10-4c1/2$0[
(-)’
1
zF$OHP
2RT
sinh
(T)]
(2)
zF$OHP
where u, (C/cm2)is the charge density at the sorbent/solution interface arising from sorption of TBA+ or NBS-, C, (F/cm2)
Q95
is the specific capacitance of the compact part of the electrical double layer, (volts) is the interfacial potential, c is the ionic strength, Z = +1, F is the Faraday constant (96487 C/equiv), T = 298 K, R is the ideal gas constant (8.314 C.V/(equiv.K)), and +ow (volts) is the potential at the outer Helmholtz plane (OHP). In the present study the interfacial excess, ri,of the sorbed organic ion i* is measured for various bulk solution concentrations of if as a function of ionic strength by the column equilibration technique. The value of uo can be obtained from rias described in the next paragraph. The interfacial potential $, is constant for a constant activity of the potential-determining ion, if, in bulk solution, regardless of ionic strength. Under conditions where $o is constant eq 2 predicts a simple straight line plot of
The slope of the plot is (2.28 x 104$o)-1 and the ratio of the intercept to the slope is 2.28 X 10-4C1-1.From these Go and C1can be evaluated. Applying eq 2 to the experimental data is not straightforward because uo does not have a simple relationship to the experimentally measured ri. The interfacial charge density arises only from that amount of interfacial excess of i’ which is sorbed a t the interface, rkD: Q,
= ZiFriAD
(3)
where Zi is +1 for TBA+ and -1 for NBS-. However, the experimentally measured ri includes all of the ion i* in the double-layer region, both that which is sorbed and, in a negative sense, that which has been expelled from the diffuse layer. ri can be converted to riAD by an iterative process involving the relationship (4)
described in detail in refs 10 and 11. For the NBS- data presented below $OHp is always small so that the parenthetic term in eq 4 is equal to unity. For the TBA+ data $OHp generally has larger values and the parenthetic term in eq 3 varies from 1.05 to 1.33 in the extremes. ODS Sorbent. If residual silanol groups are totally absent from the ODS sorbent, then the process responsible for sorption of a solute ion will be either adsorption onto the ODS/mobile phase interface (36)or partitioning into the ODS pseudoliquid phase (37). Regardless of whether the sorption process is adsorption or partition, the electrical double-layer model is applicable. However, to the extent that there are residual silanol groups present, deviations from the doublelayer model would be expected because there would be cation-exchange sites and because the interface would have a negative electrical charge in the absence of adsorbed sample ions due to the (slight) ionization of the silanols to form silanolates. To minimize residual silanol groups a highly end-capped bonded phase was used. Assuming that the parent silica gel possessed about five surface silanol groups per square nanometer (38)and that it has been 95% derivatized and endcapped in the ODS packing, as claimed by the manufacturer, then there should be about 4 x lo-” mol of residual silanol groups/cm2 (i.e. about 1 X lo4 mol of silanol groups/g) of ODS packing. It is, of course, unlikely that this number is truly the number of residual silanol groups. It is, however, the number of accessible residual silanol groups. If sample species such as nitrobenzene, which is commonly used with an alkane
996
ANALYTICAL CHEMISTRY, VOL.
63,NO. 10, MAY 15, 1991
Table I. Evaluation of the Sorption Isotherms for TBA+ According to the Freundlich and Langmuir Equations' ionic strength
Freundlichb
0.050 0.070 0.100 0.300
0.94 f 0.04 1.01 f 0.02 1.09 f 0.02 1.09 f 0.05 1.19 f 0.04
0.500
Freundlich* b
10lOa 0.180 0.179 0.189 0.160 0.157
f 0.008 f 0.005 f 0.004 f 0.010
A 0.006
LangmuiP Freundlichbsc IO%TBA, r
0.997 0.999 0.999 0.994 0.997
Langmuird - 2.3 X
10ll(rWA-
mol/cm2
Limo1
1.58 2.38 2.30 4.55 4.70
f 0.08 f 0.09 f 0.11 f 0.45 f 0.29
3.02 3.05 3.34 3.65 4.42
lo-"),
Langmuied area per TBAtr r nm2/ion 0.999 0.998 0.999 0.990 0.994
f 0.14 f 0.08 f 0.12 f 0.19
f 0.12
3.12 3.10 2.94 2.79 2.47
f 0.08 f 0.05 f 0.06 f 0.09 0.04
*
vs In [TBA+];see eq 5. r = correlation coefficient 'Uncertainties are standard deviations. bFrom linear regression of a plot of In vs [TBAt]-'; see eq 6. eFor comparison,cross-sectional area of linear regression. dFrom linear regression of a plot of (rTBA - 2.3 X of TBA+ ion is 0.5 nm2.
eluent as a test molecule (39),and TBA+, which is used in the present study, are sterically prevented from getting close enough to adsorb onto all but 4 x IO-" mol/cm2 of residual silanol groups on the end-capped polymeric ODS packing, then, as far as its sorbent properties we concerned, Partisil-10 ODS-3 can be considered to possess only about 5% residual silanols. During the several months over which all of the experiments on TBA+ (small precolumn) or on NBS- (large precolumn) were performed, the same portions of packing were left in the precolumns. Since high concentrations of surfactants such as TBA+ have been reported to decrease the lifetime of ODs-packed columns, the stability of the small precolumn was checked by periodically measuring the amount of acetaminophen adsorbed from a 2.08 X 10-5M aqueous solution in a column equilibration experiment. Over this period of time the relative standard deviation of the amount adsorbed was 6% and there was no trend in the amount adsorbed, either up or down. Sorption of TBA+.A number of workers have studied the sorption of TBA+ on ODS packings, and some have reported that it is difficult to completely elute TBA+ with eluents that contain no electrolyte (40,41). This is presumably because anionic silanolate groups, arising from ionization of residual silanol group, constitute a cation exchanger. For this reason electrolyte was included in the eluent for TBA+ in the present study and, also, it was considered necessary to demonstrate unequivocally by the INNA experiment that TBA+ was quantitatively eluted. In the INNA experiment, the ODS packing was found, after elution, to contain (1.9 f 8.3)X lo-* mol of the brominated derivative TBABr+/g of packing (Le., 6x mol/cm2). Since the smallest concentration of TBA+ loaded onto the packing in any of the isotherm experiments was 8 X 10-5 mol/g, it is evident that, even in the worst case, a t least 99.9% of the sorbed TBA+ was washed off in the elution step. A second potential complication in the TBA+ system was also ruled out. I t was demonstrated by measuring surface tension as a function of concentration of the chloride salt, TBA+Cl-, up to 0.020 M and finding no break in the curve that, even at ionic strengths as high as 0.5, no micelles form (27, 42). This conclusion is consistent with the findings of other workers for the bromide salt TBA+Br- (43,44)and with theoretical predictions (45). The sorption isotherms of TBA+ a t five different ionic strengths are presented in Figure 1. Other workers have previously reported sorption isotherms for TBA+ onto ODS packings and onto related octylsilyl (c8)packings. They usually have attempted to classify the observed isotherms on the basis of their shape as either Freundlich or Langmuir isotherms. For sorption from water/acetonitrile (9:l) onto an ODS packing, the isotherm followed the Freundlich equation (19). For sorption from water/acetonitrile (75:25) onto a c8 packing, the isotherms followed the Langmuir equation (46, 47). For sorption from methanol/water at
0.5
w -
E
6
0.3 0.1 0.07 0.05
3 5 E
W
4
;;I,
3
3
3
c
I
0 0.0
0.5
,
I
1.0
.
,
1.5
.
,
.
2.0
, 2.5
[TBA'] x lo2 (mol/L) Figure 1. Sorption isotherms for TBA' on PartisiClO ODs-3 from pH 5 aqueous solution at five ionic strengths. The numbers by the curves indicate the ionic strength.
constant ionic strength (48) and from water without control of ionic strength (49)onto ODS packing, the isotherms were found to follow neither the Freundlich nor the Langmuir equation. Stahlberg (26) summarized the results of other studies and showed that, for his own data on sorption from a pH 2.1 buffer/acetonitrile (9:l) mixed solvent onto ODS packing, the isotherms followed a modified Langmuir equation. The Langmuir equation was modified according to the approach of Stern (5) to take into account the changing electrical potential of the interface. The variability among the measured isotherm shapes reported in the literature is probably due to variations in the nature of the bonded phase (i.e. monomeric versus polymeric), in the number and degree of ionization of residual silanol groups, and in the completeness of elution of TBA+ (e.g. whether or not the eluent contained electrolyte, as discussed above). The isotherms reported in the present study were obtained by bringing the packing to complete equilibrium with the TBA+ solution and by completely eluting the sorbed TBA+. They exhibit interesting properties. The isotherms, shown in Figure 1,do not follow the Langmuir equation but do follow the Freundlich equation: ln
rTBA
= b In [TBA+]
+ In a
(5)
where a and b ( b < 1) are constants (42). Summarized in Table I are the values of the Freundlich constants along with the correlation coefficient obtained by linear regression of In [TBA+] upon In T'TBA. The isotherms in Figure 1 all possess a very steep rise up to r T B A = 2.3 X mol/cm2, followed by a more gradual ascent and possible approach toward a limiting upper value. Interestingly, if the value Z3X IO-" mol/cm2 is subtracted
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
from every point, the five resulting isotherms follow the Langmuir equation:
I
3.5 1 0.002
'uc
s 1 8
0.004
3.0 -
0.006
E u
0.010
W
in which & - B A is an equilibrium constant and r m A , m = is the limiting maximum interfacial excess which would be achieved at high solution concentration. For each of the five ionic strengths, linear regression of [TBA+]-' upon (I'TBA - 2.3 x 1O-l1)-l yields a very good straight line (not shown) with values of KTBA,( r T B A , m m - 2.3 X IO-"), and correlation coefficient as shown in Table I. Also presented in Table I are the interfacial areas occupied per TBA+ ion at saturation, calculated from the five values of rTBA,max. For comparison, if TBA+ is taken as a sphere, its projected surface area is 0.5 nm2/ion. The significance of observing a Langmuir isotherm should not be overinterpreted. Many systems which do not conform to the Langmuir assumptions nevertheless display experimental isotherms that conform to the Langmuir equation, especially if studied over a limited range of concentrations (9,42). The discussion in this section is intended, in part, to show that, depending on the experimental conditions employed, such as the mobile-phase pH and composition and the composition of the eluent used to strip an ODS packing, the observed isotherm for TBA+ might follow the Freundlich equation, the Langmuir equation, or perhaps some combination of both (and, therefore, neither alone), all of which have been reported in the literature. Effect of Silanols on TBA+. Other workers have reported that the amount of TBA+ sorbed on reversed-phase bondedphase packings, such as CU, increases with increasing mobile-phase p H (40,46,47,50). It has been suggested that this phenomenon is related to the fact that cationic samples sorb more strongly onto anionic silanolate sites by the process of cation exchange than they sorb onto silanol sites or onto ODS groups and that the number of silanolate sites increases with pH. These previous studies were done by using packings that are not highly end-capped, so that about 50% of the total silanol groups on the parent silica gel remained as residual silanol groups (51),and they employed mobile phases containing organic solvent modifiers. For these reasons sorption of TBA+ onto the ODS groups constituted a smaller fraction of the total sorbed TBA+ than in the present study, in which the packing is highly end-capped and a purely aqueous mobile phase is employed. The effect of pH was investigated in the present study by measuring r T e A obtained with aqueous mobile phases that were all 2.88 X M TBA+ and c = 0.100 and which had three different pH's: 4.2, 5.3, and 6.4. Assuming that the acid-base behavior of the residual silanol groups on Partisil-10 ODs-3 is similar to that on the parent silica gel, the degree of ionization of residual silanols a t ionic strength 0.100 is approximately 0.3% a t pH 4.2, 1% at pH 5.3, and 2% at pH 6.4 (38). However, in spite of this increase in the surface concentration of residual silanols, the experimantally measured r T B A stayed the same, (3.29 f 0.05) x IO-" mol/cm2, and showed no trend either up or down over this pH range. Taking into account the above observations, a likely interpretation of the very steep rise in the TBA+ isotherms up to = 2.3 X lo-" mol/cm2, as seen in Figure 1,is that there are approximately 2.3 X lo-" mol/cm2 of residual silanol groups accessible to TBA+ and that virtually all of them are occupied by a TBA+ ion. This number of residual silanols is in agreement with the crudely estimated value of 4 X lo-" mol/cm2, which is based on a nominal 95% end-capping. The silanol group is evidently a very strong adsorption site for
997
0.014
X
1
2
ZF c-1'2[(
3
VOHP
2RT
i'l
5
4
ZFVOHP
sinh/ 2RT
I]
-1
Figure 2. Plots of TBA' data from Figure 1 at five activities of TBA+ according to eq 2, to show agreement with the SGC model. The numbers by the curves indicate the activities of TBA'.
TBA+ from this aqueous mobile phase. The amount of TBA+ sorbed above rmA = 2.3 X lo-" mol/cm2 presumably is sorbed by the ODS reversed-phase groups themselves, and its sorption follows the Langmuir equation. The small degree of ionization of the silanol groups expected at pH 5 suggests that the vast majority of that portion of TBA+ sorbed onto residual silanol groups is adsorbed onto the neutral silanol sites and only a very small amount is present as a result of cation exchange on silanolate sites. This view is consistent with the results of titration studies of a conventional ODS packing which led to the conclusion that it is only a very small fraction of the residual silanols which are "highly acidic" and available for cation exchange at pH's commonly used in reversed-phase chromatography (52) and with the results of selective elution studies of another conventional ODS packing which showed that only about 0.05% of the total residual silanols can sorb TBA+ by cation exchange a t pH i= 5 (53). Thus,in the present study, virtually all TBA+ is sorbed onto neutral sites, whether it is sorbed onto residual silanols or onto the ODS groups, and therefore all contribute to the surface charge density, uo. SGC Behavior of TBA+. It is obvious from Figure 1 that the extent of sorption of TBA+ onto the ODS packing increases with ionic strength. This effect is predicted from SGC theory and can be quantified via eq 2 using the following five steps. (i) The ionic activity coefficients of TBA+ in the aqueous phase were used to convert molar concentrations, [TBA+], into activities, amA. Activity coefficients for TBA+ a t ionic strength greater than 0.1 have been measured (54), and those a t lower ionic strength were calculated from the extended Debye-Huckel equation (55)by assuming an ionic size parameter of 3.0 x lo-* cm. (ii) At a fixed actiuity of TBA+ a value of r m A was read from each of the five isotherms in Figure 1, corresponding to the five ionic strengths. This was repeated for five different uTBA values. (iii) A value of u, was calculated via eq 3 for each of the 25 values of r T B A by means of the iterative calculations discussed above. (iv) For each of the five aTBA values a plot was made of a;' versus c-lI2 [(ZFJ.oHp/2RT)-' sinh (ZFJ.oHP/2RT)J-'. These straight-line plots are presented in Figure 2. (v) For each straight line the values of $o, C1, and r were calculated. They are presented in Table 11. The fact that the plots in Figure 2 are straight lines constitutes proof that eq 2, and therefore the SGC theory, correctly accounts for the sorption behavior of TBA+ on the ODS packing. The capacitance C1 is a constant, independent of both amAand ionic strength, as predicted by SGC theory, with
QQe
ANALYTICAL CHEMISTRY, VOL. 63,NO. 10, MAY 15, 1991
~
~~~
Table 11. SGC Behavior of TBAt on the ODS Packing from sinh Linear Plots of u;l vs C-~~'[(ZF$~~~I~RT)-~ ( ZF$OAp/2RT)]-1at Five Activities of TBAt in Solution QmA,mol/L 0.002 0.004 0.006 0.010 0.014
mean
0.30
P
lo3$,, V b
106C1,F/cm2
0.25
0.994 0.991 0.994 0.993 0.992
106 f 13 107 f 17 125 f 16 154 f 20 169 f 21
59 f 9 68 f 13 60 f 10 50 f 7 47 f 7 57 f 9'
0.20 0.15
0.10 0.05
r = correlation coefficients of linear regression of plots in Figure 2. buncertainties of values of $, and C1 are standard deviations. cUncertainty of mean value of C1 is a "pooled" standard deviation. (I
0.00
0.00
0.05
0.10
0.15
0.20
[NBS-] x l o 2 (mollL) Sorption isotherms for NBS- on Partisil-10 O D s 3 from pH aqueous solution at five ionic strengths. The numbers by the curves indicate the ionic strength.
Figure 4. 5
Table 111. Evaluation of the Sorption Isotherms for NBSAccording to the Langmuir Equation as a Plot of I'ms-I vs [NBS-I-l' ionic strength 0.010 0.022 0.034 0.050 0.100
Plot of the results for TBA+ from Table I1 accordlng to the Nernst equation. The line has a slope of 59.2 mV/decade. Figure 3.
a value of 57 f 9 KF/cm2, The thickness of the compact part of the double layer can be calculated from C1to be 10 f 2 A. This is in the same range as, though higher than, the value of 3 f 1A estimated for the radius of a hydrated C1- ion (551, which, in principle, establishes the capacitor thickness (10). Part of the reason for the thickness of the compact layer being greater than the C1- radius may be the fact that the TBA+ ions which are adsorbed at silanol sites lie closer to the silica surface than do the TBA+ ions which are sorbed by the reversed-phase ODS groups. Plotted in Figure 3 is J., (from Table 11) versus log UTBA. The line drawn through these points has a slope of 59.2 mV/decade. Thus, within experimental error, the potential-determining TBA+ ion exhibits a Nernstian slope. Similar behavior has previously been demonstrated for the adsorption of the cation diphenylguanidinium (DPGH+) on the nonpolar, macroporous adsorbent Amberlite XAD-2 (IO). Though Nernstian behavior is not a necessary criterion for applying SGC theory, many systems in which ions are sorbed a t interfaces do exhibit this type of ideal behavior (3, 9). What is implied by the Nernstian slope is that TBA+ is the major ion responsible for the interfacial charge and that any other ionized species a t the interface are present in much smaller amounts. Sorption of NBS-. Sorption isotherms of NBS- a t five different ionic strengths are presented in Figure 4. All five of these isotherms follow the Langmuir equation over their entire measured lengths. This is demonstrated by the fact that plots of rNBS-l vs [NBS-I-' (not shown) are very good straight lines. Presented in Table I11 are the values of Km, rNM", r, and the area occupied per NBS- ion a t interfacial saturation, obtained from the Langmuir plots. The limiting areas, a t all c, are far greater than the cross-sectioned area
10-2KNBs, 10i1rNB9, mol/cm2 L/mol 7.5 f 0.8 6.4 f 0.5 4.3 f 0.5 4.5 f 0.3 2.5 f 0.6
0.26 f 0.03 0.37 f 0.03 0.59 f 0.07 0.63 f 0.04 1.1 f 0.2
area per NBS-," rb
nm2/ion
1.00 1.00 1.00 1.00 1.00
64 f 7 45 f 4 28 f 3 26 f 2 15 k 3
a Uncertainties are standard deviations. r = correlation coefficient of linear regression. e For comparison, cross-sectional area of NBS- is 0.9 nm2.
Table IV. SGC Behavior of NBS- on the ODS Packing from Linear Plots of u0-I vs c-1/2at Five Activities of NBSin Solution aNBS,
mol/L
0.0002 0.0004 0.0006 0.0008 0.0010
mean
P
0.995 0.998 0.997 0.998 0.998
-103$,,
vb
2.54 f 0.15 4.34 f 0.05 5.20 f 0.04 5.88 f 0.03 5.93 f 0.05
106C1,F/cm2 40 f 5 44 f 4 59 f 6 71 f 7 106 f 20 64 f 23b
r = correlation coefficients of linear regression of plots in Fig ure 5. buncertainties of values of $, C1, and mean of C, are
standard deviations.
of an NBS- ion, which is about 0.9 nm2. An explanation for the low saturation coverage of the interface by NBS- may be found in the fact that the interface is negatively charged due to surface silanoate sites. This causes both electrostatic repulsion of NBS- and perhaps steric blockage of the surface (38).
SGC Behavior of NBS-. The isotherm data in Figure 4 were quantitatively evaluated in terms of the SGC model by five steps, analogous to those described for TBA+. Ionic activity coefficients for NBS- were obtained from the table of Kielland (55) by assuming an ionic size parameter of 3.5 x lo* cm. When values of uo were calculated from those of r N B s by means of the iterative calculation, it turned out that because r N B s was always relatively small, the term [(ZFJ.oHp/2RT)-1 sinh (ZFJ.oHp/2R7')]-1in eq 2 had the value 1.00 under all conditions. Therefore, for each uNBS a plot of uo-I was made versus The linear plots are presented in Figure 5, and values of $, C,, and r and are presented in Table IV.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
h I
1
8 2 Eu
v
“b
r(
r 0.0008 0.0010
X
2
4
6
8
10
12
c-1/2 Flgure 5. Plots of NBS- from Figure 4 at five activities of NBSaccording to eq 2, to show agreement with the SGC model. The numbers by the curves indicate the activities of NBS-.
The linearity of the plots in Figure 5 establishes the fact that the SGC theory does account for the sorption behavior on the ODS packing. The capacitance C1has an average value of (64 f 23) X lo* F/cm2 over the range of a- studied. The variability of C1, though greater than that found in the TBA+ system, is comparable to what was found for adsorption of DPGH’ on Amberlite XAD-2 (10). The thickness of the compact part of the double layer is calculated from C1 to be 11 f 4 A, which is comparable to, though slightly larger than, the value of 4 f 1 8, reported for a hydrated Na+ ion (55)) which, in principle, establishes the capacitor thickness (10). In the NBS- system the silanolate ions, the NBS- ions adsorbed on neutral silanol groups, and the NBS- ions sorbed by the ODs bonded phase are presumably located at three different average distances from the silica gel surface, which may account for the somewhat greater than expected empirical thickness for the compact layer. Effect of Silanols on NBS-. The following evidence suggests that the values of -$, in Table IV are erroneously low due to the effect of residual silanol groups: A plot of $, versus log aNBs(plot not shown) is linear (r = 0.999) but has a slope of only -5.0 f 0.4 mV/decade, which is less than 10% of a Nernstian slope. This suggests that the sorbed NBS- ion is not the only source of negative charge at the interface but that there is another source, which is independent of am (10). The silica gel surface is the most likely source. It is important to note that, the following discussion notwithstanding, it is still true that the linearity of the plots of u0-I vs c-1/2in Figure 5 stands as a valid test of the applicability of the SGC model to describe sorption of NBS-on the ODS packing, even though the values of u, and $, will prove to be wrong. Some workers view the development of a negative surface charge on silica gel as ionization of the silanol groups to form silanolate (38,56), while others view it as adsorption of OHions (38). In either view it is correct to identify the OH- ion in solution as a potential-determining ion. In the absence of other potential-determining ions such as NBS- or TBA+ and at constant p H the surface potential $, arising from the surface charge on silica gel would be constant and independent of ionic strength because the activity of potential-determining ion is constant. A t constant pH the quantities u,, $om,and c would be related to one another by eq 2. In the present case both NBS- and OH- are potential-determining ions. The true value of interfacial charge density should be given, not by eq 4, but by 00
= zF(rNBSAD
+ roHAD)
(7)
in which rOHm is the surface excess of silanolate or hydroxide
QQQ
on the silica gel surface. Thus, the five plots in Figure 5 are each made at constant values of $o, as required, because thay are made at constant pH and constant uNBS, but the values calculated for u0-I on the vertical axis are too high because they do not include the contribution of r0HAD. The quantity roHADis constant at constant pH and c , and it is larger at larger c. The magnitude of roHADis not known but can be estimated by assuming that the residual silanols have similar acid-base behavior to silanols on the parent silica gel. At pH 5.00 the degree of ionization is expected to vary from about 0.4% at c = 0,Ol to about 1 % at c = 1.0 (38). Thus, if one assumes that the total residual silanols on the packing are as small as 5% (Le. 24 X10-l1 mol/cm2) of the initial silanols, then the value of roHADwould be in the range 2 X to 4X mol/cm2. (Of course if the total percent of residual silanols, accessible and nonaccessible, is higher than 5% then FORAD is even higher.) Comparison of these values of roHm with the experimental values of rm on the sorption isotherms in Figure 4 shows that roHm is comparable in value to rm. Thus, neglect of roHm when u, is calculated from eq 7 produces erroneously low values of u,. In the plots of u;l vs c-Il2 (Figure 5), the absolute values of both the slopes and intercepts are erroneously large by the extent to which u0-I is too large. As a consequence, the calculated values of -$, are erroneously low but those of C1are correct because the latter are calculated from the ratios of slopes and intercepts. Incidentally, because the sums ( r N s s A D + roHAD)are rather small, it would still be an acceptable approximation to take the bracketed term in the denominator on the right-hand side of eq 2 as having the value 1.00. According to this view, the markedly sub-Nernstian slope is analogous to the sub-Nernstian response of ion-selective electrodes in the presence of two different potential-determining ions, one of which is present in solution a t constant activity (57). The plot of q0vs In a m seems to be linear rather than noticeably curved because it is nearly horizontal compared to a line with Nernstian slope and because the range of a m covered is only 1 order of magnitude. This behavior is similar to that previously reported for sorption of the cation benzylammonium (BZH+) on a reversed-phase, styrene-divinylbenzene adsorbent ( I O ) . There too, when uo was calculated from rBzHAD, a linear plot of u,,-l vs c-ll2 was obtained but a much less than Nernstian dependence of $, on log u B was observed.
CONCLUSIONS Ionic strength dependence of the surface excess of sorbed ion, while not the only way to test for SGC behavior, provides an established, convenient, and legitimate criterion to use for this purpose. In these studies it has been demonstrated that the sorption of organic cations (e.g. TBA+) and of organic anions (e.g. NBS-) onto an ODS bonded phase can be quantitatively explained in terms of the classical SGC model, which has been used for years to describe the sorption behavior of ions at interfaces. In a subsequent paper (27,58) the simultaneous sorption of a low concentration of NBS- as sample ion and a higher concentration of TBA+ as “pairing ion” is investigated to experimentally verify the SGC-based model for so-called “ion-pair“ chromatography, which we previously have proposed (11).
ACKNOWLEDGMENT We thank Shandra Angle of Energy, Mines, and Resources (Canada) for providing the BET measurement of surface area on our sample of Partisil-10 ODS-3 and Ross Chow of the Alberta Research Council for allowing us to use the Wilhelmy balance. Registry No. TBA+, 10549-76-5; NBS-, 30904-42-8.
~
1000
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
LITERATURE CITED (1) Karger, B. L.; Le Page, J. N.; Tanaka, N. I n High Performance Liquid Chromatography: Advances and Pwspectives; Horvath, C., Ed.; Academlc Press: Toronto, 1980: Vol. 1, Chapter 3, pp 149-164. (2) Ion Pair Chromatography: Theoly and Biological and pharmaceutical Appllcations; Hearn, M. T. W., Ed.; Mercel Dekker: New York, 1985. (3) Hough, D. 8.; Rendall, H. M. I n Adsorption from Solutlon at the SolMILiquid Interface: Parfltt, G. D., Rochester, C. H., Eds.; Academic Press: Toronto, 1983; Chapter 6. (4) Rosen, M. J. Surfactants and Interfacial Phenomena; Wlley: New York, 1978; Chapter 2. (5) Grahame, D. C. Chem. Rev. 1947, 4 1 , 441-501. (6) Hiementz, P. D. Rlncipbs of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1966; Chapter 12. (7) Shew, D. J. Introduction to Collokl and Surface Chemistry, 3rd ed.; Butterworths: Toronto, 1980 Chapter 7. (8) Davles, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York. 1961; Chapter 4. (9) Lyklema, J. I n Absorption from Solution at the SolidlLiquid Interface: Parfitt. 0. D., Rochester. C. H.. Eds.; Academic Press: Toronto, 1983; Chapter 5. (10) Cantwell, F. F.: Puon, S. Anal. Chem. 1979, 51, 623-832. (11) Cantwell, F. F. J . Warm. Biomed. Anal. 1984, 2 , 153-164. (12) Weber, S. G.; Orr, J. D. J . Chromatogr. 1985, 322, 433-441. (13) Bedard. P. R.: Purdy, W. C. J . Liq. Chfomatogr. 1985, 8 , 2417-2443. (14) Cantwell, F. F. I n Ion Exchange and Solvent Extraction: A Series of Advances; Marinsky, J. A.; Marcus, Y., Eds.; Marcel Dekker: New York, 1985; Vol. 9, Chapter 6. (15) Hammers, W. E.: Aussems, C. N. M.; Janssen, M. J. Chromatow. 1988, 360, 1-12. (16) Lafosse, M.; Dreux, M.; Morin-Allory, L. J . Chromatogr. 1988, 368, 273-282 -. - -- - . (17) Stahlberg, J. J. Chromatogr. 1988, 356, 231-245. (18) Pletrzyk, D. J.; Iskandaranl, 2 . ; Schmltt, G. L. J . Liq. Chromatogr. 1988, 9 , 2633-2659. (19) Del Ray, M. E.; Vera-Avlla, L. E. J . Liq. Chromatogr. 1987, 10, 2911-2929. (20) Stahlberg, J.; Furangen, A. Chromatographla 1987, 2 4 , 783-789. (21) Stahlberg, J. Chromatographla 1987, 2 4 , 820-826. (22) Cope, M. J.; Davldson. I.E. Anaiyst 1987, 112. 417-421. (23) Mullins, F. G. P. Analyst 1987, 112, 665-671. (24) Brandts, P. M.; Gelsema, W. J.; De Llgny, C. L. J . Chromatogr. 1988, 438, 181-195. (25) Weber, S. G. Talanta 1989, 36, 99-106. (26) Stahlberg, J.: Hagglund, 1. Anal. Chem. 1988, 60, 1958-1964. (27) Liu, H.J. Ph.D. Thesis, Unlversity of Alberta, Fall, 1988. (28) May, S.; Hux, R. A.; Cantwell, F. F. Anal. Chem. 1982, 5 4 , 1279-1282. (29) Hux, R. A.; Cantwell, F. F. Anal. Chem. 1984, 56, 1258-1263. (30) Technical Bulletln, LC-111-6183: Whatman. Inc.: Clifton, NJ, 1985. (31) Gustavli, K.; Schlll, G. Acta Pharm. Suec. 1988, 3 , 241-258.
(32) Fossey, L.; Cantwell, F. F. Anal. Chem, 1982, 5 4 , 1693-1697. (33) Hiementz. P. C. Principles of Colloidand Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986; Chapter 6. (34) Bauer, H. H.; Christian, G. D.; OReilly. J. E. Insfru;mentelAnalysiJ; 2nd ed.; Allyn and Bacon: Boston, 1978; Chapter 19. (35) Barnhurst, J. D. J . Org. Chem. 1981, 26, 4520-4522. (36) Colin, H.; Gulchon, G. J . Chromatogr. 1978. 158, 183-205. (37) Martire, D. E.; Bohem, R. E. J . W y s . Chem. 1983, 87, 1045-1062. (38) . . Iler, R. K. The Chemistry of Silica; Wlley: New York, 1979; DD .. 355-360, 657-687. (39) Sander, L. C.: Wise. S. A. CRC Crit. Rev. Anal. Chem. 1987, 18, 299-415. (40) Van der Houwen, 0. A. G. J.; Sorel, R. N. A.; Hulshoff, A.; Teeuwsen, J.: Indemans, A. W. M. J . Chmmatogr. 1981, 209. 393-404. (41) Sorel, R. H. A.; Hulshoff, A. I n Advances in Chrmtcgmphy; GW dings, J. C., Grushka, E., Cazes, J., Brown, P. R., Eds.; Marcel Dekker: New York, 1983; Chapter 3. (42) Hiementz, P. C. Principles of Colloidand Surface Chemistry, 2nd ed.; Marcel Dekker: New York. 1986; Chapter 7. (43) Arnett, E. M.: Ho, M.: Schaleger, L. L. J . Am. Chem. Soc.1970, 92, 7039-7042. (44) Tamaki, K. Bull. Chem. SOC.Jpn. 1987, 40, 38-41. (45) Nandi, P. K. J . Phys. Chem. 1974, 78, 1197-1202. (46) Tllly-Melin, A.; Ljungkrantz, M.; Schlll, G. J . Chromatogr. 1979, 185, 225-239. (47) Tllb-Melin. A.; Askemark, Y.; Wahlund. K. G.: Schlll, 0. Anal. Chem. 1979, 51, 978-983. (48) Bartha, A.; Vigh, Gy. J . Chromatogr. 1983, 260, 337-345. (49) Hung, C. T.; Taylor, R. B. J . Chromatogr. 1980. 202, 333-345. (50) Gelilkens, C. F.; De Leenher, A. P. J. Chromarogr. 1980, 194, 305-3 14. (51) Unger, K. K. Porous Silica-Its Roperties and Use as a Support In Column Liquid Chromatography; Elsevier: Amsterdam, 1979; pp 130- 133. (52) Weber, S. G.; Tramposch, W. G. Anal. Chem. 1983, 55, 1771-1775. (53) Smith, R. L.; Iskandarani, 2.; Pletrzyk, D. J. J . Liq. Chfomatogr. 1984, 7, 1935-1959. (54) Llndenbaum, S.; Boyd. G. E. J . Phys. Cham. 1964, 68, 911-917. (55) Klelland, J. J. Am. Chem. SOC. 1937, 5 9 , 1675-1878. (56) Healy, T. W.; WhRe, L. R. Adv. Colloid. Interface Sci. 1978, 9 . 303-345. (57) Ross, J. W. I n Ion Selective Electrodes; Durst, R. A,, Ed.; NBS Special Publication 314; U.S. Government Printing Office: Washington, DC, 1969; Chapter 2. (58) Liu, H.J.; Cantwell, F. F. University of Alberta, unpublished results.
RECEIVED for review September 4,1990. Accepted February 15,1991. This work was supported by the Natural Sciences and Engineering Research Council of Canada and by the University of Alberta.
